The present invention relates to a melt tension enhancer for polyolefin resins that contains (A) polytetrafluoroethylene and (B) a polymer based on an alkyl (meth)acrylate of 5-30 carbon atoms, and to a process for its production.
Polyolefin resins have been widely used in the past for a variety of molded products because of their low cost and excellent physical properties. However, because of low tension upon melting (hereunder referred to as xe2x80x9cmelt tensionxe2x80x9d) in the case of polypropylene, for example, there are some disadvantages in terms of processability, including inferior vacuum moldability, blow moldability, foam moldability, extrusion moldability and calender moldability.
Polyethylene and the like are often blended with polypropylene toward the aim of improving these processability, but since the improvement in processability is insufficient, large amounts of polyethylene are required and this leads to the disadvantage of lower rigidity of the resulting blend. It has been attempted to raise the melt tension by increasing the molecular weight of the polyolefin, but higher molecular weight is a problem because it reduces the melt flow property which is one parameter of the processability, thus making it impossible to achieve a suitable balance between the melt tension and the melt flow property.
As a polyolefin resin with improved processability there has been disclosed polypropylene having a free-ended long-chain branched structure, in Japanese Unexamined Patent Publication No. 62-121704, Japanese Unexamined Patent Publication No. 2-298536 and elsewhere. The unique viscoelasticity of this polypropylene allows it to maintain the strength of resin films during foam molding, thus making it possible to produce foams with highly independent cells that have not been possible with conventional straight-chain polypropylene. However, because this type of polypropylene requires a special treatment method or synthesis method involving electron beam irradiation or peroxide addition in order to produce the free-ended long-chain branched structure, it has the disadvantage of greatly increased production costs for the resin.
On the other hand, metallocenes that work with methylaluminoxane as a co-catalyst have high uniformity in terms of activity compared to conventional catalysts, exhibit excellent copolymerization properties, and give polyethylene with a narrow molecular weight distribution and composition distribution at a high activity. Polyolefins obtained by metallocene catalysts have excellent heat seal properties and hot tackiness, but their narrow molecular weight distribution results in a low melt tension and they are hence known to have problems in terms of molding processability; an improvement in melt tension, therefore, is still desired.
Polytetrafluoroethylene has high crystallinity and low intermolecular force and therefore has the property of becoming fibrous under slight stress, while its combination with thermoplastic resins provides improved molding processability and mechanical properties, so that it has come to be used as an additive for thermoplastic resins.
For example, Japanese Unexamined Patent Publication No. 5-214184 and Japanese Unexamined Patent Publication No. 6-306212 disclose resin compositions comprising polyolefins added to polytetrafluoroethylene. Also, Japanese Unexamined Patent Publication No. 7-324147 discloses a process for production of a polyolefin resin composition obtained by mixing polytetrafluoroethylene and a dispersing medium powder under high shear, wherein a polyolefin is combined therewith after first rendering the polytetrafluoroethylene fibrous. In addition, Japanese Unexamined Patent Publication No. 9-25420 discloses a process that uses polytetrafluoroethylene encapsulated with styrene/acrylonitrile copolymer to improve the melting rate of various resins such as polyvinyl chloride resin.
However, polytetrafluoroethylene has the disadvantage of poor dispersability in common thermoplastic resins including no halogen atoms, and as taught in Japanese Unexamined Patent Publication No. 5-214184 and Japanese Unexamined Patent Publication No. 6-306212, it fails to uniformly disperse by simple blending and thus notably lowers the surface appearance of molded products.
Even with the process of Japanese Unexamined Patent Publication No. 7-324147, it is difficult to render all of the polytetrafluoroethylene fibrous by shear force, and the fibrous polytetrafluoroethylene therefore also aggregates in the matrix resin making it impossible to obtain a homogeneous composition.
Moreover, while the process of Japanese Unexamined Patent Publication No. 9-25420 attempts to improve affinity with the matrix resin by encapsulation, there is no effect of improved dispersability in polyolefin resins.
In other words, all of these processes leave the problem of dispersability of polytetrafluoroethylene in polyolefin resins, with the disadvantages of requiring large amounts of polytetrafluoroethylene to exhibit the useful properties mentioned above, and giving molded products with impaired surface appearance.
It is an object of the present invention to provide a melt tension enhancer that increases the dispersability of polytetrafluoroethylene in polyolefin resins and improves the molding processability of polyolefins without impairing the surface appearance of molded products.
As a result of diligent research aimed at overcoming the problems discussed above, the present inventors have completed the present invention upon finding that the melt tension of a polyolefin resin can be improved without impairing the surface appearance of molded products, by adding to the polyolefin resin a resin composition containing polytetrafluoroethylene and a polymer based on an alkyl (meth)acrylate of 5-30 carbon atoms.
The present invention therefore provides a melt tension enhancer for polyolefin resins comprising (A) polytetrafluoroethylene and (B) a polymer based on an alkyl (meth)acrylate of 5-30 carbon atoms, and to a process for its production.
As examples of polyolefin resins to be used for the invention there may be mentioned resins wherein the main component is a homopolymer of an olefin monomer or a copolymer of olefin monomers obtained by radical polymerization, ion polymerization or the like, a copolymer of a dominant amount of an olefin monomer and a minor amount of a vinyl-based monomer, or a copolymer of an olefin monomer and a diene-based monomer, and any of these may be used alone or in combinations of two or more. The polymerization catalysts used for these resins may be known ones such as Ziegler catalysts, chromium catalysts or metallocene catalysts.
As the olefin monomer referred to here there may be mentioned ethylene, propylene, 1-butene, 1-hexene, 1-decene, 1-octene, 4-methyl-1-pentene and the like, among which ethylene and propylene are particularly preferred. As specific examples of homopolymers or copolymers of these olefinic monomers there may be mentioned low-density polyethylene, very low-density polyethylene, very very low-density polyethylene, linear low-density polyethylene, high-density polyethylene, ultrahigh molecular weight polyethylene, polypropylene, ethylene-propylene copolymer, polymethylpentene, polybutene and the like. These olefinic polymers may be used alone or in combinations of two or more. Particularly preferred among these are polyolefin resins whose main components are mixtures of one or more selected from the group consisting of polyethylene, polypropylene and ethylene-propylene copolymer.
The (A) polytetrafluoroethylene in the melt tension enhancer of the invention may be obtained by polymerization of a monomer component composed mainly of tetrafluoroethylene by a known process. The (A) polytetrafluoroethylene may contain, as copolymerized components, fluorine-containing olefins such as hexafluoropropylene, chlorotrifluoroethylene, fluoroalkylethylenes, perfluoroalkylvinyl ethers, etc. and fluorine-containing alkyl (meth)acrylates such as perfluoroalkyl (meth)acrylates, so long as the original properties of the polytetrafluoroethylene are not diminished. The content of copolymerized components is preferably no greater than 10 wt % based on the polytetrafluoroethylene.
The (B) polymer based on an alkyl (meth)acrylate of 5-30 carbon atoms in the melt tension enhancer of the invention may be obtained by polymerization of a monomer component comprising an alkyl (meth)acrylate of 5-30 carbon atoms by radical polymerization, ion polymerization or the like. As specific examples of alkyl (meth)acrylates of 5-30 carbon atoms there may be mentioned cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, octadecyl (meth)acrylate and isobornyl (meth)acrylate. These monomers may be used alone or in combinations of two or more.
As monomers that are copolymerizable with the alkyl (meth)acrylate of 5-30 carbon atoms, there may be mentioned styrene-based monomers such as styrene, p-methylstyrene, o-methylstyrene, p-chlorstyrene, o-chlorstyrene, p-methoxystyrene, o-methoxystyrene, 2,4-dimethylstyrene and xcex1-methylstyrene; alkyl (meth)acrylate monomers of 1-4 carbon atoms such as methyl (meth)acrylate, ethyl (meth)acrylate and butyl (meth)acrylate; vinyl cyanide monomers such as acrylonitrile and methacrylonitrile; vinyl ether monomers such as vinyl methyl ether and vinyl ethyl ether; vinyl carboxylate monomers such as vinyl acetate and vinyl butyrate; olefin monomers such as ethylene, propylene and isobutylene; and diene monomers such as butadiene, isoprene, dimethylbutadiene, etc. These monomers may also be used alone or in combinations of two or more.
The melt tension enhancer of the invention contains the (A) polytetrafluoroethylene and (B) polymer based on an alkyl (meth)acrylate of 5-30 carbon atoms, where the weight ratio of (B)/(A) is preferably 0.2-100, and more preferably 0.5-50. If the weight ratio of (B)/(A) is less than 0.2 the dispersability of the polytetrafluoroethylene may be reduced. At greater than 100, the effect of the polytetrafluoroethylene may not be obtained.
The amount of the polytetrafluoroethylene in the melt tension enhancer of the invention is preferably 0.05-40 wt % based on the total weight of the melt tension enhancer. At less than 0.05 wt % it will have to be added in too large amount to achieve sufficient melt tension, and the rigidity and heat resistance of the polyolefin resin may be impaired. At greater than 40 wt % the dispersability of the polytetrafluoroethylene may be reduced.
By combining this melt tension enhancer in an amount such that the polytetrafluoroethylene content is 0.001-20 parts by weight per 100 parts by weight of the polyolefin resin, it is possible to obtain a polyolefin resin composition with satisfactory moldability, having the polytetrafluoroethylene component homogeneously dispersed in the polyolefin resin in a fine fibrillated state, and providing improved melt tension without impairing the surface appearance of molded products.
The melt tension enhancer of the invention can be obtained as a powder by a first process in which an aqueous dispersion of polytetrafluoroethylene particles with a particle size of 0.05-1.0 xcexcm is combined with an aqueous dispersion of particles of a polymer based on an alkyl (meth)acrylate of 5-30 carbon atoms with a particle size of 0.05-1.0 xcexcm, and the mixture is allowed to aggregate or spray-dried. It can also be obtained as a powder by a second process in which a monomer with an ethylenically unsaturated bond is polymerized in a dispersion containing a mixture of polytetrafluoroethylene particles with a particle size of 0.05-1.0 xcexcm combined with an aqueous dispersion of particles of a polymer based on an alkyl (meth)acrylate of 5-30 carbon atoms with a particle size of 0.05-1.0 xcexcm, and the product is then allowed to aggregate or spray-dried.
The aqueous dispersion of polytetrafluoroethylene particles used to produce the melt tension enhancer of the invention can be obtained by emulsion polymerization of a monomer component composed mainly of tetrafluoroethylene.
As representative commercially available starting materials for the polytetrafluoroethylene particle dispersion there may be mentioned Fluon AD-1 and AD-936 (trademarks) by Asahi ICI Fluoropolymers, Polyflon D-1 and D-2 (trademarks) by Daikin Industries, and Teflon 30J (trademark) by Mitsui-DuPont Fluorochemicals.
The aqueous dispersion of particles of the polymer based on an alkyl (meth)acrylate of 5-30 carbon atoms used to produce the melt tension enhancer of the invention may be obtained by polymerization of a monomer component comprising an alkyl (meth)acrylate of 5-30 carbon atoms by a known emulsion polymerization process or mini-emulsion polymerization process.
There are no particular restrictions on the ethylenically unsaturated bond-containing monomer that is further polymerized in the dispersion containing the mixture of the aqueous dispersion of polytetrafluoroethylene particles with a particle size of 0.05-1.0 xcexcm combined with the aqueous dispersion of particles of a polymer based on an alkyl (meth)acrylate of 5-30 carbon atoms with a particle size of 0.05-1.0 xcexcm, in the second production process for the melt tension enhancer of the invention, and it may be selected from among styrene-based monomers such as styrene, p-methylstyrene, o-methylstyrene, p-chlorstyrene, o-chlorstyrene, p-methoxystyrene, o-methoxystyrene, 2,4-dimethylstyrene and xcex1-methylstyrene; (meth)acrylate monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, cyclohexyl (meth)acrylate, glycidyl (meth)acrylate and 2-hydroxyethyl (meth)acrylate; vinyl cyanide monomers such as acrylonitrile and methacrylonitrile; vinyl ether monomers such as vinyl methyl ether and vinyl ethyl ether; vinyl carboxylate monomers such as vinyl acetate and vinyl butyrate; olefin monomers such as ethylene, propylene and isobutylene; and diene monomers such as butadiene, isoprene, prene, dimethylbutadiene, etc. These monomers may also be used alone or in combinations of two or more.
The addition of the melt tension enhancer of the invention to the polyolefin resin may be accomplished by melt kneading by a known process such as extrusion kneading or roll kneading. Alternatively, it may be combined in a multistage process whereby the melt tension enhancer of the invention is combined with a portion of the polyolefin resin to prepare a master batch, after which the remainder of the polyolefin resin is added and mixed therewith.
The polyolefin resin composition containing the melt tension enhancer of the invention has increased tension when melted, improves the drawing properties upon calendering, the draw down of melted resins for thermoforming or blow molding, and the open cell formation upon foam molding, and improves the general processability for calendering, thermoforming, blow molding, foam molding and the like. It also improves the discharge volume for extrusion molding and the surface condition of extrusion molded articles such as sheets and films, as well as the extrusion processability. There is also no macro-aggregation of the polytetrafluoroethylene, so that the surface appearance of the molded products is excellent.
A filler may also be added to the polyolefin resin composition to which the melt tension enhancer of the invention has been added. The filler content is preferably 0.1-400 parts by weight per 100 parts by weight of the polyolefin resin composition, and inclusion of a filler can improve the rigidity and heat resistance, improve the processability such as the calenderability to prevent adhesion onto roll surfaces, and achieve cost reduction. At less than 0.1 part by weight the effect of improved rigidity may be insufficient, and at greater than 400 parts by weight the surface quality may be reduced. As representative fillers there may be mentioned calcium carbonate, talc, glass fiber, magnesium carbonate, mica, kaolin, calcium sulfate, barium sulfate, titanium white, white carbon, carbon black, ammonium hydroxide, magnesium hydroxide, aluminum hydroxide and the like. Preferred among these are calcium carbonate and talc.
If necessary, the polyolefin resin composition to which the melt tension enhancer has been added may further contain other additives such as a stabilizer, lubricant, flame retardant or the like. Representative examples of each that may be mentioned include, as stabilizers, phenolic stabilizers such as pentaerythrityl-tetrakis [3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] and triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], phosphorus-based stabilizers such as tris(monononylphenyl) phosphite and tris(2,4-di-t-butylphenyl) phosphite, and sulfur-based stabilizers such as dilaurylthiodipropionate; as lubricants, sodium, calcium or magnesium salt of lauric acid, palmitic acid, oleic acid or stearic acid; and as flame retardants, phosphoric acid ester compounds including trimethyl phosphate, triethyl phosphate, tributyl phosphate, trioctyl phosphate, tributoxyethyl phosphate, triphenyl phosphate, tricresyl phosphate, cresylphenyl phosphate, octyldiphenyl phosphate, diisopropylphenyl phosphate, tris(chloroethyl) phosphate, polyphosphates such as alkoxy-substituted bisphenol A bisphosphate, hydroquinone bisphosphate, resorcin bisphosphate and trioxybenzene triphosphate; tetrabromobisphenol A, decabromodiphenyl oxide, hexabromocyclododecane, octabromodiphenyl ether, bistribromophenoxyethane, ethylene bistetrabromophthalimide, tribromophenol, halogenated compounds including various halogenated epoxy oligomers obtained by reacting halogenated bisphenol A and epihalohydrin, carbonate oligomers with halogenated bisphenol A as a structural component, halogenated polystyrenes, chlorinated polyolefins and polyvinyl chloride, as well as metal hydroxides, metal oxides, sulfamine oxides, and the like.
The method of processing the polyolefin resin composition to which the melt tension enhancer of the invention has been added may be a method such as calendering, thermoforming, blow molding, foam molding, extrusion molding, injection molding or melt spinning.
The polyolefin resin composition to which the melt tension enhancer of the invention has been added can be used to obtain useful molded products including sheets, films, thermoforms, hollow molds, foams, extrusion molded products, fibers, and the like.
The present invention will now be explained in further detail by way of examples, with the understanding that the invention is in no way limited to these examples.
Through the following descriptions, xe2x80x9cpartsxe2x80x9d and xe2x80x9c%xe2x80x9d are by weight.
The measurements of the different properties given in the Reference Examples, Examples and Comparative Examples were conducted as described below.
(1) Solid concentration: The particle dispersion was dried at 170xc2x0 C. for 30 minutes.
(2) Particle size distribution and weight-average particle size: The particle dispersion was diluted with water as a sample solution, and measured by the dynamic light scattering method (ELS800 by Otsuka Denshi, KK. (trademark), temperature: 25xc2x0 C., scattering angle: 90xc2x0).
(3) Zeta potential: The particle dispersion was diluted with a 0.01 mol/l NaCl aqueous solution as a sample solution, and measured by the electrophoresis method (ELS800 by Otsuka Denshi, KK., temperature: 25xc2x0 C., scattering angle: 10xc2x0).
(4) Melt tension: A pellet of the polyolefin resin composition was extruded at a constant extrusion rate (10 mm falling speed/min) using a falling flow tester (Capillograph (trademark) by Toyo Seiki), the strand was pulled at a constant rate (4 m/min), and the melt tension was measured. The L/D of the die was 10.0 mm/xcfx862.0 mm, and the measuring temperature was 200xc2x0 C.
(5) Swelling ratio: A pellet of the resin composition was extruded at a constant extrusion rate (1.5 mm falling speed/min) using a falling flow tester (Capillograph by Toyo Seiki), the strand diameter (D) was measured at a position 5 mm under the nozzle, and calculation was made by the equation below. The L/D of the die was 10.0 mm/xcfx862.0 mm, and the measuring temperature was 190xc2x0 C. when polypropylene was included and 160xc2x0 C. for polyethylene alone.
(Swelling ratio)=D (mm)/2.0
(6) Melt flow rate: A pellet of the resin composition was used for measurement with a 2.16 kg load according to ASTM D1238. The measuring temperature was 230xc2x0 C. when polypropylene was included and 190xc2x0 C. for polyethylene alone.
(7) Elastic modulus: A pellet of the resin composition was injection molded into a test piece which was measured according to ASTM D790.
(8) Roll sheet appearance: A pellet of the resin composition was used, and the outer appearance of a roll sheet during roll kneading was visually judged.
∘: No irregularity of surface, excellent gloss.
xcex94: Some irregularity of surface, slightly poor gloss
X: Considerable irregularity of surface, poor gloss
(9) Draw down: A pellet of the resin composition was used for molding of a 100-mm square sheet with a thickness of 1.5 mm which was then anchored with a clamp having a 76-mm square opening, and the draw down of the sheet over 30 minutes was measured in an oven at 190xc2x0 C.
(10) Evaluation of foam molded products: 1.0 part of azodicarbonamide (foaming agent) was added to 100 parts of a pellet of the resin composition, and injection molding was performed to create a foam molded product, upon which the condition of the cross-sectional cells was visually judged.
∘: Fine and uniform
xcex94: Somewhat non-uniform
X: Non-uniform
(11) Condition of foam sheets: The surface condition of the foam sheet and its cross-section were judged by observation.
∘: No surface irregularities or continuous air bubbles, with an overall homogeneous condition.
X: Scaly irregularities or corrugations on the surface, or open cell structural sections, with no overall homogeneous condition.
(12) Calendering temperature range: A twin roll (calender roll) with a 15.24 cm diameter was used under conditions with a roll surface temperature in the range of 160-230xc2x0 C., a roll speed of 10 m/min (roll speed ratio=1:1.1) and a roll spacing of 0.3 mm, for a procedure in which a 100 g sample was contacted onto the rolls for 5 minutes and a sheet was drawn out, and the temperature range giving a satisfactory sheet was determined.
(13) Calenderability property: The release property from the rolls and the surface quality were evaluated in the calendering of (12) above.
Release property
∘: Released with no resistance
xcex94: Released with resistance
X: Difficult to draw out sheet
Surface quality
∘: No roughness
xcex94: Fine roughness
X: Large roughness
(14) Bleeding property: The sheet obtained in (12) above was used to make a 1-mm thick press sheet, and after allowing it to stand for one week in a hot oven at 60xc2x0 C., the degree of bleeding of the sheet surface was evaluated.
∘: No bleeding
xcex94: Some bleeding
X: Considerable bleeding
(15) Total light transmittance, haze and gloss (45xc2x0 gloss) of sheets: These were measured by the method described for JIS K7105.
(16) Moldable temperature range for sheets: A sheet was molded (diameter: 10 mm, height: 6 mm) at a constant pressure using an air-pressure forming machine (FBP-M2 (trademark) by CKD Co.), and the ideal moldable temperature range giving a molded product with a uniform overall thickness was determined.